Wireless communications, such as cellular telephony, have gained widespread acceptance in the personal communications marketplace. More recently, digital communications methods used in personal communications systems have proven useful as well. Digital systems offer superior performance in the presence of noise, and permit the implementation of time division schemes, allowing for more efficient use of available frequency spectrum.
Among the various digital communications systems, quadrature amplitude modulation (QAM) and .pi./4 quadrature phase shift keying (QPSK) have proven to be particularly useful. Many such systems employ what is referred to as cartesian, or I-Q modulation. In I-Q modulation, both the inphase and quadrature components of a carrier are varied. Since many of the devices using digital modulation techniques are hand held, battery powered devices, it is desirable to make them as efficient as possible to extend the life of the battery. Modulation schemes utilizing amplitude modulation require linear amplification of signal in the transmitter to prevent distortion and interference of adjacent channels. Linearity and efficiency are conflicting goals for a conventional power amplifier. One technique that has improved linearity over conventional schemes is the use of cartesian feedback, as is known in the art. An exemplary treatment of the topic can be found in an article titled "Transmitter Linearization Using cartesian Feedback For Linear TDMA Modulation", by M. Johansson and T. Mattsson, as published in the proceedings of the 41st IEEE Vehicular Technology Conference, St. Louis, USA, VTC-91, pp 439-444, May 1991.
In designing a cartesian feedback loop, as in any feedback system, the open loop gain is an important parameter. If the gain is too high, the loop will be unstable; if it is too low, not enough corrective effect will be achieved. The gain is typically tuned and set to a desired level when the device is assembled. This is slow, and requires special test instruments to perform. There are at least two significant problems in designing the device so that the gain is at an optimum level. First, since such devices are manufactured in large numbers, and are often sold in consumer markets, the cost of the device is a critical market factor. Therefore, it is desirable to manufacture such devices with as few custom parts as possible. Building the device with "off the shelf" components, however, results in a significant variance in certain parameters, such as gain. Accordingly, care is taken in designing the loop so as to account for such variance from one unit to the next, and assure proper operation despite component value drift during operation. A second problem results from the fact that these communication devices can experience changes in temperature and voltage during operation, resulting in drift of operating parameters. However, through careful engineering, reasonable variation may be accounted for with a robust design. Unfortunately, design robustness necessarily increases product cost in both materials and engineering time.
It is desirable to avoid this cost, if possible, without compromising design performance. One way to achieve this goal is to design the transmitter such that the transmitter can somehow tune the gain of the power amplifier periodically. Therefore there is a need, in a transmitter using cartesian feedback, for a means by which the transmitter can tune the gain of the power amplifier to maintain feedback loop stability and achieve the desired corrective effect of feedback.